Effects of heavy-ion irradiation on the microwave surface impedance of (Ba1−x K x )Fe2As2 single crystals

The electrodynamic response of Ba1−xKxFe2As2 single crystals at the microwave frequencies has been investigated by means of a coplanar resonator technique, at different values of nonmagnetic disorder introduced into the samples by heavy-ion irradiation. The surface impedance Zs = Rs + iXs conforms to the classical skin effect above the critical temperature. Below Tc, Rs monotonically decreases while Xs shows a peak, which evolves as a function of the irradiation fluence. The disorder-dependent Zs(T) curves are analyzed within a two-fluid model, suitably modified to account for a finite quasiparticle fraction at T = 0. The analysis gives, for the unirradiated crystal, quasiparticle relaxation times τ that are in good agreement with previous literature. Smaller τ values are deduced for the disordered crystals, both in the normal and in the superconducting states. The limits of application of the model are discussed.